A synchronous digital transport network operates for example in accordance with the recommendations of the ITU-T (Telecommunication Standardization Sector of International Telecommunication Union) for SDH (Synchronous Digital Hierarchy) or SONET (Synchronous Optical Network), useful information being packed in so-called containers. The containers contain an overhead section known as “path overhead” together with which they are referred to as virtual containers VC-N. There are virtual containers of the types VC-11, VC-12, VC-2, VC-3 and VC-4. There are also contiguously concatenated containers of the types VC-4-4c and VC-4-16c in the case of SDH, and VC-3-3c, VC-3-12c and VC-3-48c in the case of SONET. The virtual containers represent multiplex units and are multiplexed in a frame referred to as synchronous transport module STM-N (N-1, 4, 16 or 64), the virtual containers being able to be arbitrarily positioned in the payload section of the transport modules and addressed by a pointer in the overhead section of the transport modules. Smaller virtual containers here are always multiplexed in larger virtual containers. A synchronous transport module in SDH for example always comprises a virtual container VC-4, or in the case of contiguously concatenated containers, a VC-4-nc (n=4 or 16). On the other hand, in the case of SONET each transport module always contains three VC-3s, and in the case of contiguously concatenated containers a VC-3-3c, which corresponds to a VC-4, a VC-3-12c or a VC-3-48c. The described multiplex hierarchy of SDH and SONET is described in detail in ITU-T G.707 (3/96) Chapter 6.
Communications signals transmitted in a synchronous transport network of this kind are thus frame-structured, synchronous multiplex signals being composed of frames each having a payload section and an overhead section. Multiplex units are multiplexed in the payload section of each transport frame in accordance with the multiplex hierarchy specified by the ITU-T. The overhead section contains a pointer to the largest multiplex unit contained in the payload section and one section referred to as RSOH (regenerator section overhead) and one section referred to as MSOH (multiplex section overhead). These contain items of check- and control information which in the case of the RSOH are terminated and regenerated by each regenerator and in the case of the MSOH by each multiplexer. Amongst other things, management functions of the network management system are performed with the aid of these items of check-and control information. Increasingly, synchronous transport networks are being used not only by public network operators but also as company networks with company network management systems. If a plurality of SDH- or SONET-based sub-networks of a company network are to be connected via the transport network of a public operator, different use of the check- and control information from RSOH and MSOH can lead to conflicts between the network management systems of company transport network and public transport network. Agreements on a uniform use of the check- and control information are necessary and the network management system of the company transport network is restricted with regard to the management of its company network. Thus for example the multiplex structure cannot be changed from 16×VC-4 to 1×VC-4-16c in a STM-16 without the agreement of the public network operator.
Optical networks via which so-called optical channels (OCh) are to be transmitted are also currently in development. These networks are to be capable of transparently transmitting signals with any bit rate and therefore these future optical networks will also be capable of transparently transporting frame-structured communications signals from a company network. However as yet no binding standard exists for this purpose and moreover the construction of such an optical network would require the replacement of all the network elements.